Selectively removing H.sub.2 S from a gas stream containing H.sub.2 S and CO.sub.2 is a process common to many industries such as the coke-making, the petroleum and the natural gas industries. Of the several techniques employed one utilizes the selective absorption of H.sub.2 S into a wide variety of liquid absorbent solutions which can then be regenerated to afford a concentrated H.sub.2 S gas stream for further processing in a sulfur recovery system. In this type of process aqueous alkanolamines are the most widely used absorbent solution. The H.sub.2 S and CO.sub.2 containing gas stream, such as coke oven gas, is usually treated with the alkanolamine solution in any suitable absorption apparatus such as a packed bed absorber, a spray contact apparatus, a bubble-cap tray absorber and the like. The H.sub.2 S will react almost instantaneously with the aqueous alkanolamine solution to form alkanolammonium sulfide or hydrosulfide which may then be decomposed by the application of heat to the solution to strip the H.sub.2 S from the absorbent solution, for example by the use of steam in a desorption stage.
Carbon dioxide, on the other hand, takes a significantly finite time to react with the water in the alkanolamine solution to form carbonic acid according to the well-known equilibrium reaction prior to reacting with the alkanolamine to form alkanolammonium carbonate or bicarbonate. Thus the CO.sub.2 does not tend to be taken up by the alkanolamine solution as readily, and is consequently not removed from the gas stream as quickly as the H.sub.2 S. In general, it may be simplistically stated that the molar ratio of H.sub.2 S to CO.sub.2 absorbed in the absorbent solution will depend principally upon the gas-liquid contact volume and the gas residence time in this absorption volume. Since H.sub.2 S has a much greater rate of absorption into an alkaline absorbent than does CO.sub.2 and if the absorption is conducted under non-equilibrium conditions in a manner such that the gas stream is only in contact with the absorbent solution for a relatively short period of time (i.e., relative to the time for an equilibrium condition to be established), then some selectivity for H.sub.2 S can be obtained if larger amounts of H.sub.2 S can be tolerated in the treated gas stream than would otherwise be present with a longer contact time. So long as the flow rate of the gas to the absorber is relatively constant and is significantly greater than the time required to absorb all of the CO.sub.2 into the solution, the ratio of H.sub.2 S to CO.sub.2 absorbed will tend to remain substantially constant. The unabsorbed CO.sub.2 leaves the absorption apparatus along with any other unabsorbed gases which may be present as the exhausted desulfurized gas.
Subsequent to the absorber is the desorption apparatus which thermally drives the absorbed gases out of the absorbent solution to yield regenerated absorbent for recycling to the absorber and an H.sub.2 S and CO.sub.2 containing desorbed acid gas stream which is directed to a sulfur recovery plant, such as a Claus plant.
The major problem of the above described H.sub.2 S removal and sulfur recovery system is the limited flexibility of the process. When the feed gas flow rate decreases in a given absorption system, the contact or residence time of the gas in the absorbent solution immediately increases with consequent lowering of the selectivity because the absorption process moves in the direction of equilibrium conditions for the absorption of CO.sub.2 and results in a decreasing H.sub.2 S:CO.sub.2 absorption ratio.
Upon desorption, the increased CO.sub.2 content of the desorbed gas stream may reach such a concentration that it will not support the combustion of the H.sub.2 S in the sulfur recovery plant which then becomes inoperative.
One solution to this problem of inflexibility is offered in a copending application Ser. No. 877,639 titled "Selective Removal of H.sub.2 S from an H.sub.2 S and CO.sub.2 Containing Gas Stream" by J. A. Laslo, and D. Kwasnoski, filed concurrently with and assigned to the assignee of this application. Laslo and Kwasnoski disclose that the H.sub.2 S concentration of the desorbed acid gas stream relative to the CO.sub.2 concentration can be maintained above the H.sub.2 S:CO.sub.2 ratio at which the downstream Claus type sulfur recovery system becomes inoperative when the flow rate of the H.sub.2 S and CO.sub.2 containing feed gas stream decreases by contacting the feed gas with the absorbent solution in a smaller absorption volume. This smaller absorption volume is sized to be a percentage of the original absorption volume which is within a percentage point range of the decreased gas flow rate at which the sulfur recovery system becomes inoperative or at which the switch to the smaller absorption volume is made, expressed as a percentage of the maximum gas flow rate. By performing the gas-liquid contact in a smaller volume the reduced gas flow will have a lesser residence time in this smaller absorption volume than it would have in the original absorption volume which is dimensioned to treat the maximum gas flow. The lesser residence time means a higher H.sub.2 S:CO.sub.2 ratio in the absorbent solution.
A solution to the broader problem of selectively removing a desired gaseous component from a variably flowing gas stream by absorption into a liquid absorbent solution which will absorb at least one additional less desired gaseous component from the gas stream at a different absorption rate is disclosed by J. A. Laslo, M. O. Tarhan, C. W. Sheldrake and D. Kwasnoski in a copending application Ser. No. 877,634 titled "Selective Removal of a Gaseous Component from a Multi-Component Gas Stream" filed concurrently with, and assigned to the assignee of, this application. Laslo et al. pass the two fluid streams countercurrently through a total absorption volume which comprises at least two individual absorption volumes connected in series. The passage of one of the fluid streams can be regulated through the individual absorption volumes in such a manner as to effectively add or subtract individual absorption volumes in which gas-liquid contact occurs in response to the variations in the gas flow rate thus achieving a more constant residence time. A more constant residence time means the absorbent solution will contain dissolved in it a ratio of desired component to less desired component which is more nearly like that ratio for which the absorption system was designed.
Laslo et al. show that good flexibility and turndown can be achieved by having a plurality of individual absorption volumes compose the total absorption volume. Sequentially adding or subtracting individual absorption volumes in the series in a cumulative manner can be accomplished rather easily. There is, however, a problem with incorporating or removing individual absorption volumes in a non-sequential cumulative manner from the middle of the connected series. While it can be accomplished with a plurality of individual volumes serially connected in one absorption volume, the valves, piping and hardware necessary render such an embodiment unattractive.
Accordingly, there is a need in a total absorption volume comprising individual absorption volumes to have the capability of adding or removing individual absorption volumes in a non-sequential manner. Particularly, there is a need to provide a total absorption volume having the increased flexibility to treat a variable gas flow more efficiently and economically by using the maximum possible absorption capacity of a selected combination of individual absorption volumes where the combination may be one other than a solely sequential series. More particularly, there is a need for an H.sub.2 S absorber that changes its absorption volume capacity in response to the varying flow rate of an H.sub.2 S and CO.sub.2 containing gas stream to maximize the H.sub.2 S content of the desorbed acid gas and thus optimize the operation of the sulfur recovery plant.